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Introduction to two-temperature equations of state

Description: A brief introduction to two-temperature equations of state is presented. Current capabilities of T-4 in this area as well as plans for future development are discussed. In addition, we point out various conceptual and practical difficulties in implementing a two-temperature approximation.
Date: July 1, 1979
Creator: Albers, R.C.
Partner: UNT Libraries Government Documents Department

LMTO (Linear Muffin-Tin Orbital) electronic band-structure calculations for linear Pt-Br chains

Description: Self-consistent first-principles Linear Muffin-Tin Orbital (LMTO) scalar-relativistic and Dirac-relativistic electronic band structure calculations are presented for linear Pt-Br chains (without any ligand structure) embedded in a three-dimensional crystal with empty atomic spheres added to isolate the chains. These calculations are a first step towards a complete a priori calculation of the electronic structure of the halogen-bridged linear platinum chain compound Pt/sub 2/Br/sub 6/(NH/sub 3/)/sub 4/, an MX chain compound (M = transition metal and X = halogen atoms). 10 refs., 4 figs.
Date: January 1, 1988
Creator: Albers, R.C.
Partner: UNT Libraries Government Documents Department

Brillouin-zone integration schemes: an efficiency study for the phonon frequency moments of the harmonic, solid, one-component plasma

Description: The efficiency of four different Brillouin-zone integration schemes including the uniform mesh, special point method, special directions method, and Holas method are compared for calculating moments of the harmonic phonon frequencies of the solid one-component plasma. Very accurate values for the moments are also presented. The Holas method for which weights and integration points can easily be generated has roughly the same efficiency as the special directions method, which is much superior to the uniform mesh and special point methods for this problem.
Date: January 1, 1981
Creator: Albers, R.C. & Gubernatis, J.E.
Partner: UNT Libraries Government Documents Department

Magnetic properties and electronic structure of the actinide systems UAl/sub 2/, NpAl/sub 2/, and PuAl/sub 2/

Description: The electronic ground states of UAl/sub 2/, NpAl/sub 2/ and PuAl/sub 2/ have been determined by band theory techniques. These calculations indicate that both actinide d-f hybridization and spin-orbit coupling are important in these systems. Using a new scheme for obtaining the paramagnon parameters for UAl/sub 2/ it is shown that calculated and experimentally derived 5f band width are consistent. The fact that orbital angular momentum may not be quenched in these systems is suggested as a possible mechanism for damping spin-fluctuations in NpAl/sub 2/ and PuAl/sub 2/.
Date: January 1, 1985
Creator: Albers, R.C.; Boring, A.M. & Koelling, D.D.
Partner: UNT Libraries Government Documents Department

Electrically inactive poly-silicon grain boundaries

Description: Structures, energies, and electronic properties of symmetric [001] tilt grain boundaries in Si have been studied using Stillinger-Weber and Tersoff classical potentials, and semi-empirical (tight-binding) electronic structure methods. The calculated lowest energy (310) grain boundary structure and electronic properties are consistent with previous TEM measurement and calculations. For the controversial (710) grain boundaries, the tight-binding calculations do not show any electronic energy levels in the band gap. This indicates that with every atom fully fourfold coordinated, the (710) grain boundary should be electrically inactive. Some high-energy metastable grain boundaries were found to be electrically active by the presence of the levels introduced in the band gap. Also, the vacancy concentration at the (310) GB was found to be enhanced by many orders of magnitude relative to bulk. The dangling bond states of the vacancies should be electrically active.
Date: May 1, 1996
Creator: Chen, S.P.; Kress, J.D.; Voter, A.F. & Albers, R.C.
Partner: UNT Libraries Government Documents Department

First-principles study of intermetallic phase stability in the ternary Ti-Al-Nb alloy system

Description: The stability of bcc-based phases in the Ti-Al-Nb alloy system has been studied from first-principles using a combination of ab-initio total energy and cluster variation method (CVM) calculations. Total energies have been computed for 18 binary and ternary bcc superstructures in order to determine low temperature ordering tendencies. From the results of these calculations a set of effective cluster interaction parameters have been derived. These interaction parameters are required input for CVM computations of alloy thermodynamic properties. The CVM has been used to study the effect of composition on finite-temperature ordering tendencies and site preferences for bcc-based phases. Strong ordering tendencies are observed for binary Nb-Al and Ti-Al bcc phases as well as for ternary alloys with compositions near Ti{sub 2}AlNb. For selected superstructures we have also analyzed structural stabilities with respect to tetragonal distortions which transform the bcc into an fcc lattice. Instabilities with respect to such distortions are found to exist for binary but not ternary bcc compounds.
Date: December 31, 1996
Creator: Asta, M.; Ormeci, A.; Wills, J.M. & Albers, R.C.
Partner: UNT Libraries Government Documents Department

Theoretical studies of Ni/sub 3/Al and NiAl with impurities

Description: Intermetallic compound has been extensively studied because of their superior properties in strength, low creep rate, and high melting point. But most of the systems have room temperature ductility problems, like Ll/sub 2/ and B2 compounds. Both Ll/sub 2/ Ni/sub 3/Al and B2 NiAl exhibit intergranular fracture mode. Understanding grain boundaries in these materials is of particular importance since intergranular fracture limits the applicability of these otherwise promising material. In an effort trying to understand the fracture mechanism, we have used embedded atom potentials to study the properties of Ni/sub 3/Al and NiAl. We also consider the effect of boron, sulfur, and nickel segregation on the strength of grain boundaries in Ni/sub 3/Al and NiAl. 22 refs., 2 figs.
Date: January 1, 1988
Creator: Chen, S.P.; Voter, A.F.; Boring, A.M.; Albers, R.C. & Hay, P.J.
Partner: UNT Libraries Government Documents Department

Effects of B and S on Ni3Al grain boundaries

Description: This report describes the procedure used to fit embedded atom style potentials to the pure elements Ni, Al, B and S and the cross potentials necessary for treating alloys and segregants. Grain boundary simulations, and results from molecular statics simulations using the potentials for B and S, and their cross potentials with Ni and Al, which was fit from electronic band structure calculations, are included. 21 refs., 1 fig., 3 tabs. (JL)
Date: January 1, 1988
Creator: Voter, A.F.; Chen, S.P.; Albers, R.C.; Boring, A.M. & Hay, P.J.
Partner: UNT Libraries Government Documents Department

First principles total energy study of NbCr{sub 2} + V Laves phase ternary system

Description: The C15 NbCr{sub 2} + V Laves phase ternary system is studied by using a first-principles, self-consistent, full-potential total energy method. Equilibrium lattice parameters, cohesive energies, density of states and formation energies of substitutional defects are calculated. Results of all these calculations show that in the C15 NbCr{sub 2} + V compounds, V atoms substitute Cr atoms only.
Date: April 1, 1999
Creator: Ormeci, A.; Chen, S.P.; Wills, J.M. & Albers, R.C.
Partner: UNT Libraries Government Documents Department

Alloy design and phase stability of the ternary alloy titanium-aluminum-niobium

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The authors have used high-precision electronic-structure and cluster-variational method (CVM) techniques to study aspects of alloy design involving the phase stability and ordering tendencies of the ternary system Ti-Al-Nb, and the correlation with these to changes in the electronic structure of these materials. This system is of great interest for aerospace applications due to its desirable mechanical properties and low densities. Total energies were computed for 18 binary and ternary bcc superstructures in order to derive parameters for CVM calculations, which showed important strong ordering tendencies in the alloy phase diagram as a function of temperature and alloy concentration. Structural optimization calculations were used to analyze structural instabilities for bcc, fcc, hcp, O-phase, and {omega}-phase structures. The authors discovered the mechanism for the role of Nb in the structural stability of the O-phase of the ternary intermetallic Ti{sub 2}AlNb. Calculations were also done to investigate the electronic properties associated with the structural stability of a related class of Laves Phase high-temperature structural materials NbCr{sub 2} and HfV{sub 2}.
Date: December 31, 1998
Creator: Albers, R.C.; Chen, Shao-Ping & Wills, J.M.
Partner: UNT Libraries Government Documents Department

EOS, thermodynamic, and structural-mechanical properties of intermetallic compounds

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). Several classes of binary intermetallic compounds have important potential application as high temperature structural materials because of their high melting temperature, low density, and high strength, but their use is limited by their poor low temperature ductility and fracture toughness. The goal of this project was to further the development of techniques for performing ab-initio calculations of the electronic, structural, and elastic properties of these materials in an investigation of the relation between structure, composition, and mechanical properties of intermetallics. Materials properties to be addressed in these calculations included the equation of state (EOS), defect structure energetics, and elastic constants and phonons. Major accomplishments included calculations of stacking fault and twin energies in layered TiAl, structural stability in binary and ternary Ti-Al-Nb compounds, and point defect energies and elastic moduli of Laves phase intermetallics.
Date: December 31, 1998
Creator: Wills, J.M.; Straub, G. & Albers, R.C.
Partner: UNT Libraries Government Documents Department

EXAFS Measurements of Laser-Shocked V and Ti and Crystal Phase Transformation in Ti

Description: Extended X-Ray Absorption Fine Structure (EXAFS), using a laser-imploded target as a source, can yield the properties of laser-shocked metals on a nanosecond time scale. EXAFS measurements of vanadium shocked to {approx}0.4 Mbar yield the compression and temperature in good agreement with hydrodynamic simulations and shock-speed measurements. In laser-shocked titanium at the same pressure, the EXAFS modulation damping is much higher than warranted by the predicted temperature increase. This is shown to be due to the {alpha}-Ti to {omega}-Ti crystal-phase transformation, known to occur below {approx}0.1 Mbar for slower shock waves. The dynamics of material response to shock loading has been extensively studied in the past [1]. The goal of those studies has been to understand the shock-induced deformation and structural changes at the microscopic level [2]. Laser-generated shocks can be employed to broaden these studies to higher pressures ({approx}1 Mbar) and strain rates ({approx} 10{sup 7}-10{sup 8} s{sup -1}). Recently, laser-shocked materials have been studied with in-situ x-ray diffraction [3,4]. The goal of this work is to examine the use of in-situ EXAFS [5] as a complementary characterization of laser-shocked metals. EXAFS is the modulation in the x-ray absorption above the K edge (or L edge) due to the interference of the photoelectron waves with the waves reflected from neighboring atoms. The frequency of EXAFS modulations is related to the inter-particle distance, hence to the compression. The damping rate of the modulation can yield the lattice temperature, which is not readily available by other methods.
Date: March 10, 2004
Creator: Yaakobi, B; Meyerhofer, D D; Boehly, T R; Rehr, J J; Remington, B A; Allen, P G et al.
Partner: UNT Libraries Government Documents Department

Elastic constants of a Laves phase compound: C15 NbCr{sub 2}

Description: The single-crystal elastic constants of C15 NbCr{sub 2} have been computed by using a first-principles, self-consistent, full-potential total energy method. From these single-crystal elastic constants the isotropic elastic moduli are calculated using the Voigt and Reuss averages. The calculated values are in fair agreement with the experimental values. The implications of the results are discussed with regards to Poisson`s ratio and the direction dependence of Young`s modulus.
Date: April 1, 1997
Creator: Ormeci, A.; Chu, F.; Wills, J.M.; Chen, S.P.; Albers, R.C.; Thoma, D.J. et al.
Partner: UNT Libraries Government Documents Department

Theoretical and Experimental Investigation on the Low Temperature Properties of the NbCr{sub 2} Laves Phase

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The goal of the project was to develop methodologies in which to define and improve the properties of NbCr{sub 2} so that the high temperature structural applications of alloys based upon this would not be limited by the low-temperature brittle behavior of the intermetallic. We accomplished this task by (1) understanding the defect structure and deformation mechanisms in Laves phases, (2) electronic and geometric contributions to phase stability and alloying behavior, and (3) novel processing of dual phase (Laves/bcc) structures. As a result alloys with properties that in many cases surpass superalloys were developed. For example, we have tailored alloy design strategies and processing routes in a metal alloy to achieve ambient temperature ultimate strengths of 2.35 GPa as well as ultimate strengths of 1.5 GPa at 1000 C. This results i n one of the strongest metal alloys that currently exist, while still having deformability at room temperature.
Date: June 3, 1999
Creator: Thoma, D.J.; Chu, F.; Chen, K.C.; Kotula, P.G.; Mitchell, T.E.; Wills, J.M. et al.
Partner: UNT Libraries Government Documents Department

Magnetism and bonding in actinide and rare earth systems

Description: The importance of including all of Hund's rules in self consistent electronic structure calculations is pointed out. We present a scheme that accounts for the interactions that lead to these rules, and we give examples of when these interactions become important. Calculated ground state properties (chemical bonding, magnetic moment, magnetic form factor) of actinide and rare earth systems are found to agree better with experiment when all Hund's rules are accounted for. On the other hand, for Fe, Co and Ni we find that it is a reasonable approximation to neglect Hund's second and third rule. 20 refs., 5 figs., 3 tabs.
Date: January 1, 1990
Creator: Eriksson, O.; Albers, R.C.; Boring, A.M. (Los Alamos National Lab., NM (USA)); Brooks, M.S.S. (Commission of the European Communities, Karlsruhe (Germany, F.R.). European Inst. for Transuranium Elements) & Johansson, B. (Uppsala Univ. (Sweden). Inst. of Physics)
Partner: UNT Libraries Government Documents Department

Fermi surface and effective masses for the heavy-electron superconductor UPt sub 3

Description: Local-density-approximation (LDA) calculations for the Fermi-surface extremal cross-sectional areas of UPt{sub 3} are presented and compared to deHaas-van Alphen experiments of Taillefer et al. The topology of the calculated surfaces is in excellent agreement with experiment and allows a determination of the directional dependence of the anisotropic mass-renormalization factor. The source of this renormalization is briefly discussed. 12 refs., 4 figs., 2 tabs.
Date: October 1, 1987
Creator: Norman, M.R.; Albers, R.C.; Boring, A.M.; Christensen, N.E. (Argonne National Lab., IL (USA); Los Alamos National Lab., NM (USA) & Max-Planck-Institut fuer Festkoerperforschung, Stuttgart (Germany, F.R.))
Partner: UNT Libraries Government Documents Department